Fig. 2.

a Schematic of the multichannel absorption spectrometer and pneumotachograph within the MFS measurement head. The bi-directional gas path (blue) is shown along with the two mesh screens (gray), across which pressure drop is measured. Radiation used for probing O₂ is injected into an optical cavity constructed from a pair of highly reflective mirrors (red) and collected by a photodiode (PD 1) positioned along the optical axis (green). Radiation probing CO₂ and H₂O vapor is launched into the v-path (yellow), reflected by a concave mirror, and collected by the photodiode (PD 2). The physical length of the optical cavity is commensurate with the diameter of a standard medical ventilation tube. b, c Dry gas fractions of N₂ (purple), O₂ (green), and CO₂ (red) measured in real time (every 10 ms) as a participant breathes through the measurement head. The dry fraction of N₂ is determined by the subtracting the dry fractions of O₂ and CO₂ from unity. b Data from an air breathing phase. c Data from the early stage of a nitrogen washout procedure